VCM editor cylinder volume off

[Life is an Enigma]

I've been practicing tuning using a GM 4.3 in a 2004 Chevy Blazer nice little guinea pig but I just noticed something an engine under general cylinder volume does not quite add up to the 4.3L 262Cu-in. if you do the math the cylinder volume is smaller by 0.36% so if you add a 43.5102cu-in X 6 = 261.0612 cu-in but if you do the math and divide 262 cu-in by 6 you get 43.6666 per cylinder volume. my question is is this the vcm editor interpreting the data from the PCM wrong, or did GM just make the cylinder volume smaller to correct any mistakes in the manufacturing for these engines.

[blindsquirrel]

screenshot.08-08-2023 19.03.02.png

4.012 x 4.012 x 3.48 x .7854 is actually 43.993852011648 ci/cyl, for a total displacement of 263.963112069888.

'262' (or '350', or '5.7L', for that matter) is a marketing number, not a technical specification.

[Life is an Enigma]

screenshot.08-08-2023 19.03.02.png

4.012 x 4.012 x 3.48 x .7854 is actually 43.993852011648 ci/cyl, for a total displacement of 263.963112069888.

'262' (or '350', or '5.7L', for that matter) is a marketing number, not a technical specification.

I used an engine displacement calculator and got it as close as I can. You can't get it. Perfect! For some reason in the VCM editor it will change it by one number on the last digit. After changing cylinder volume it does run better. For some reason the 4.3 does not like to run so well when it's 100 plus degrees outside they would ping especially if he had the AC on just overall gutless but after the change it runs better. The cylinder volume I finally settled on was 0.71664L or 43.7318cu-in that works for me. I know a lot of companies like to use ballpark figures sometimes but that one was definitely on the foul line. Or maybe that's what worked to make the 4.3L pass C.A.R.B & EPA emissions.

[Cringer]

I used an engine displacement calculator and got it as close as I can. You can't get it. Perfect! For some reason in the VCM editor it will change it by one number on the last digit. After changing cylinder volume it does run better. For some reason the 4.3 does not like to run so well when it's 100 plus degrees outside they would ping especially if he had the AC on just overall gutless but after the change it runs better. The cylinder volume I finally settled on was 0.71664L or 43.7318cu-in that works for me. I know a lot of companies like to use ballpark figures sometimes but that one was definitely on the foul line. Or maybe that's what worked to make the 4.3L pass C.A.R.B & EPA emissions.

This has to do with how the ECM (not HPT) saves data with ones and zero's. It has to do with how many bits are allocated to store a number. Compromises are made against accuracy and speed with regards to memory allocation, data processing, and calculation speed for all data points in the ECM optimization. So this happens for all fields in the tune, not just the field you are looking at. Some are more obvious than others due to the number size or units (grams/sec vs lbs per hour) and the significant figures 100000.000 vs .0000001.

As an example, on my Gen4 E38, the table for [ECM] 12314 - Short Pulse Adder only allows number with an interval/multiple of "0.0078125" milliseconds. So I can pick 0ms or 0.0078ms, but nothing in between. I am also limited to any multiple of .00780:
0.00780
0.01560
0.02340
0.03120
0.03900

...and so on. There is not an infinite resolution in the values.

[Life is an Enigma]

One day we're all going to look back and think this programming wasn't that hard because the future is coming and it's all EV and from what I've seen I'm not going near that with a 10 ft pole, not going try to do any of that programming.